Heavy Oil Flow Under Solution-gas Drive: Pressure Depletion Tests

Abstract

Abstract The formation of foamy oil has been postulated to be an important factor contributing to the success in primary production of heavy oil under solution-gas drive in Alberta and Saskatchewan. This paper presents the results of pressure depletion or blowdown tests on unconsolidated oil sand cores which were conducted to investigate the foamy oil flow phenomenon. Experimental results suggest that foamy oil flow is likely dominant between the bubble point and the critical gas saturation for fast tests (one-hour step duration), but may be much less important as free gas is produced via gas channels at gas saturation greater than the critical saturation. Foamy oil flow is also a function of step duration, and for slow tests (greater than 24-hour step duration), foamy oil flow may be absent. Tests also reveal that the total bitumen recovery was extremely high, ranging from 35 - 50%, and no obvious temperature effects were observed. Both features are perhaps caused by high pressure decline rates resulting from the step pressure decline. Whether foamy oil flow occurs or not, the solution-gas drive mechanisms in heavy oil reservoirs are similar to those in conventional oil. The liberation of solution gas and the expansion of free gas are the main pressure maintenance sources below the bubble point. Introduction In the primary production of heavy oil, the drive mechanism is generally believed to be solution-gas drive. It is also an important mechanism for cyclic steam stimulation. A better understanding of heavy oil reservoir performance under solution-gas drive is essential to both processes. The concept of foamy oil flow has been used to explain the anomalous performance of heavy oil reservoirs under solution-gas drive(1,2). Foamy oil flow refers to the simultaneous flow of gas bubbles and oil in a pseudo-continuous phase. Ward et al.(3) illustrated theoretically that in a closed volume, the size of the stable gas bubbles formed by a pressure drop depends upon the number of gas bubbles. The diameter of gas bubbles could be as small as 3 µm and as many as ten million bubbles could be formed in one cubic centimetre of live oil. This hypothesis prompted Smith(1) to postulate that gas bubbles could perhaps flow through the pore throat, unaffected by capillary forces. He assumed that the flow of the oil-gas mixture in the reservoir could be approximated by a single-phase flow. The average parameters for this single-phase mixture flow were estimated from the individual parameters of bitumen and gas. He reported that this single-phase model successfully predicted the pressure response observed in Lloydminster heavy oil reservoirs. Sarma and Maini et al.(4,5) studied experimentally the role of gas nucleation in the primary production of heavy oil under solution- gas drive. They concluded that the nucleation of gas bubbles was unlikely to increase the mobility of heavy oil, and that the heavy oil-gas mixture was flowing in the form of an oil-continuous foam. Poon and Kisman(6) investigated the effects of non-Newtonian fluid on the primary production of heavy oil.